Push-pull LED driver circuit

Abstract

Push-pull circuits are described that are suitable for the driving of LEDs and that reduce the voltage stress on the switching transistors that is caused by the output transformer. The push-pull arrangement caters to reducing the size of the transformer as it eliminates the DC magnetic bias of the transformer core and it also caters to the integration of the semiconductor content of the circuit requiring only low side DMOS to be implemented in the monolithic, junction isolated process.

It is claimed: 
     
       1. A driver circuit connectable to a load, comprising:
 a DC supply node; 
 first and second switches; 
 a first rectifier; 
 a transformer having first and second primary windings and a secondary winding, wherein the first and second primary windings are respectively connected to ground through the first and second switches and have a center tap connected to the DC supply node, and wherein the secondary winding is connected to the load through the first rectifier when the driver circuit is connected to drive the load; and 
 control circuitry connected to the first and second switches, whereby the first and second switches are alternately conducting when driving the load, 
 wherein, when the driver circuit is connected to drive the load, the load is connected in parallel with the first rectifier and is connected in series with the first and second switches between the DC supply node and ground through the center tap. 
 
     
     
       2. The driver circuit of  claim 1 , wherein the center tap is connected to the DC supply node through the load when the driver circuit is connected to drive the load. 
     
     
       3. The driver circuit of  claim 1 , wherein the first and second switches are connect to ground through the load when the driver circuit is connected to drive the load. 
     
     
       4. The driver circuit of  claim 1 , wherein the load is one or more LEDs connected in series. 
     
     
       5. The driver circuit of  claim 1 , wherein, when the driver circuit is driving the load, current through the load is a sum of a current from the secondary winding and a current into the center tap. 
     
     
       6. The driver circuit of  claim 1 , wherein the first rectifier includes:
 first and second series connected capacitors having an intermediate node connected to a first terminal of the secondary winding; and 
 first and second series connected diodes having an intermediate node connected to a second terminal of the secondary winding. 
 
     
     
       7. The driver circuit of  claim 6 , wherein the first and second series connected diodes are connected to the second terminal of secondary winding through a third capacitor forming a resonant circuit. 
     
     
       8. The driver circuit of  claim 1 , wherein the first rectifier includes:
 first and second series connected diodes having an intermediate node connected to a first terminal of the secondary winding; and 
 third and fourth series connected diodes having an intermediate node connected to a second terminal of the secondary winding. 
 
     
     
       9. The driver circuit of  claim 8 , further comprising:
 a capacitor connected in parallel with the first and second series connected diodes and with the third and fourth series connected diodes. 
 
     
     
       10. The driver circuit of  claim 8 , wherein the third and fourth series connected diodes are connected to the second terminal of secondary winding through a third capacitor forming a resonant circuit. 
     
     
       11. The driver circuit of  claim 1 , wherein the driver circuit further includes a second rectifier connected between the DC supply node and ground and connectable to an AC voltage source. 
     
     
       12. The driver circuit of  claim 1 , wherein the first and second switches are implemented as DMOS transistors. 
     
     
       13. The driver circuit of  claim 1 , wherein each of the first and second switches is respectively connected to ground in parallel with a corresponding one of a first diode and a second diode. 
     
     
       14. The driver circuit of  claim 1 , wherein the control circuitry uses a voltage based regulation. 
     
     
       15. The driver circuit of  claim 1 , wherein the control circuitry uses a current based regulation. 
     
     
       16. The driver circuit of  claim 1 , wherein the center tap is connected to ground through a capacitor. 
     
     
       17. The driver circuit of  claim 1 , wherein the control circuitry and first and second switches are formed on a common integrated circuit.